In working machines, in particular for agriculture and forestry as well as for road construction machines or floor conveyor vehicles, hydrostatic drives are used, preferably due to the high forces being transmittable by them and due to their robustness as well due to their low-wearness. Here, hydrostatic drives are used for propel drives as well as for propelling working devices. Preferably, hydrostatic drive mechanisms being adjustable in its displacement are used hereby in hydraulic pumps as well as in hydraulic motors. More preferably, hydrostatic devices are used, which admit a forward as well as a backward mode, a so-called reversed mode for the hydrostatic drive. For driving the hydrostatic transmission, drive engines acts mechanically on a hydraulic pump and drives the same rotatable. Preferably, the hydraulic pump is adjustable in its conveying volume such that a hydraulic fluid flow rate towards the hydraulic motor is adjustable in its amount. In order that a predetermined maximum pressure in the hydraulic lines respectively in the working lines connecting for instance the hydraulic motor with the hydraulic pump is not exceeded, pressure relieve valves are provided in branch lines splitting off from the working lines. By means of these a partial flow of hydraulic fluid can be released. With these pressure relieve valves arranged in the branch lines pressure relieve valves damage on the hydrostatic drive can be prevented in the drive mode as well as in the coasting mode, if the high pressure exceeds a maximum value.
In coasting mode, in which the hydraulic motor acts as a hydraulic pump, kinetic energy of the hydraulic transmission is transmitted, for instance, via the traction wheels of a mobile working machine to the drive mechanism of the hydraulic motor. The so driven driving mechanism of the hydraulic motor conveys, depending on the adjusted displacement (now a conveying volume), hydraulic fluid with high pressure to the hydraulic pump, which is thereby driven hydraulically. The hydraulic pump supports itself mechanically on the drive motor imposing a resistance against this driving force being denominated as well as braking—coasting—or drag moment of the driving engine. If the high pressure generated in the coasting mode by the hydraulic motor is higher than an opening pressure of a pressure relief valve adapted to the drag moment of the drive engine, a partial flow rate of the hydraulic fluid under high pressure is released over the pressure relief valves and bypasses via split lines located at the drive mechanism, the hydraulic pump. Therewith, kinetic energy of the hydrostatic drive can be transformed into heat by means of the pressure relief valve.
DE 10 2006 059 734 A1 shows such a hydrostatic drive and a method for decelerating a hydrostatic drive. In this hydrostatic drive to each working line a pressure relief valve is allocated, which releases the high pressure towards the other working line with low pressure if in the corresponding working line a high pressure is present that exceeds a predetermined value. For being usable for reversible drives also, the system of DE 10 2006 059 734 A1 shows mirror-inverted arranged pressure relief valves, each of which can be bypassed by a bypass line, whereby in each bypass line a check valve is arranged. For this, each check valve opens the bypass lines in the opposite flow direction of the pressure relief valve, to which it is allocated. The system in the aforementioned document is arranged preferably in the hydraulic pump. Due to this, the supply lines from the hydraulic motor to the pressure relief valves are very long and, further, dynamic pressures can occur in the long high pressure lines. Simultaneously during a coasting mode, in the working lines conveying low pressure, lack of hydraulic fluid can arise, which ends up in a so-called low pressure drop. Such low pressure drops occur for instance if the displacement of the hydraulic motor in coasting mode is set corresponding to the actuation/operator force on a brake actuator, whereas the hydraulic pump is displaced fixedly on a brake conveying volume. If the brake actuator is actuated stronger, the conveying volume of the hydraulic motor is increased, what possibly leads to a situation, in which the conveying volume of the hydraulic motor is bigger than the brake volume set at the hydraulic pump. In this situation the hydraulic motor does not receive the needed amount of hydraulic fluid. A low pressure drop occurs as the actual flow rate from the hydraulic pump to the hydraulic is lower than the requested one. In practice such low pressure drops are often compensated by a hydraulic fluid pressure accumulator connected to the low pressure line.